A diffusion combustion system, in which fuel and the air are ejected from different nozzles and burned, has been often used for conventional gas turbine combustors. Recently, however, a premix combustion system which is more advantageous in the reduction of thermal NOx has been also used inplace of the diffusion combustion system. The premix combustion system means that fuel and the air are premixed with each other and the mixture is ejected from the same nozzle and burned. According to this combustion system, even if fuel is rarefied, it is possible to burn the fuel in that state in any combustion regions. Therefore, it is easy to decrease the temperature of the premixed fuel, and this premix combustion system is advantageous in the reduction of NOx as compared with the diffusion combustion system. On the other hand, this premix combustion system has the following problem, that is, the stability of a combustion state is inferior because the air is excess with respect to the fuel and the temperature of premixed flames is low.
FIG. 16 is a front view which shows one example of a premix type gas turbine combustor. A combustor inner cylinder 20 is provided in a combustor outer casing 10 at a constant clearance kept between the cylinder and the casing. A diffusion flame formation cone 30 which forms diffusion flames is provided on the central portion of the combustor inner cylinder 20. The diffusion flame formation cone 30 causes pilot fuel supplied from a pilot fuel supply nozzle 31 to react with the air supplied from the portion between the combustor outer casing 10 and the combustor inner cylinder 20, and forms diffusion flames.
Although it is not clear from FIG. 16, eight premixed flame formation nozzles 40 which form premixed flames are provided around the diffusion flame formation cone 30. Premixed gas is formed by mixing the air supplied from the portion between the combustor outer casing 10 and the combustor inner cylinder 20 with main fuel and then ejected from the premixed flame formation nozzles 40. The premixed gas ejected from the premixed flame formation nozzles 40 is passed through premixed flame formation nozzle extension tubes 400 provided at the outlets of the nozzles 40, respectively and ejected toward a combustion chamber 50. This premixed gas is ignited by high-temperature combustion gas discharged from the diffusion flames and forms premixed flames. High-temperature, high-pressure combustion gas is discharged from the premixed flames, passed through a combustor tail pipe (not shown) and then introduced into the first stage nozzle of a turbine.
In the meantime, in the gas turbine combustor, the premixed gas is ejected linearly from the premixed flame formation nozzle extension tubes 400 toward the combustion chamber 50, and therefore uneven fuel concentration portions exist in the premixed gas. Therefore, combustion temperature is high on the portions of the premixed gas on which the fuel concentration is high and NOx tends to be generated on the portions. To solve this problem and to suppress further generation of NOx in the premix type gas turbine combustor, it is necessary to sufficiently mix the main fuel with combustion air. From this viewpoint, Japanese Patent Application Laid-Open (JP-A) No. 7-248118 discloses a premix combustor which is provided with a premix unit consisting of a group of cylindrical members which generate turning flows of the combustion air in a premix combustor. In addition, JP-A No. 8-28871 discloses a gas turbine combustor which is provided with units which respectively turn premixed gas in a peripheral direction so that a plurality of turning flows of the premixed gas are wound around one another or twisted into one another in the combustor.
However, in each of the gas turbine combustors disclosed in the JP-A Nos. 7-248118 and 8-28871, the premixed gas is transformed to turning flows directing inward in the direction of the center of a combustion chamber and twisted, and thereby a combustion gas recycle region cannot be sufficiently formed. Therefore, these gas turbine combustors have disadvantages in that the flame holding of the premixed flames become unstable and stable combustion cannot be thereby obtained and that oscillating combustion and the like are caused. Further, since the premixed gas is concentrated in the neighborhood of the center of the combustion chamber and combusted, local high-temperature portions tend to be generated and the generation of NOx cannot be sufficiently suppressed.